While the invention can be used generally to measure blood flow at various locations in a body, it is particularly useful in measuring blood flow in the heart so as to permit the measurement of cardiac output. Many techniques for measuring cardiac output have been suggested in the art. Exemplary thermodilution techniques described in the technical and patent literature include: "A Continuous Cardiac Output Computer Based On Thermodilution Principles", Normann et al., Annals of Biomedical Engineering, Vol. 17, 1989; "Thermodilution Cardiac Output Determination With A single Flow-Directed Catheter", Forrester, et al., American Heart Journal, Vol. 83, No. 3, 1972; "Understanding Techniques for Measuring Cardiac Outputs", Taylor, et al., Biomedical Instrumentation & Technology, May/June 1990; U.S. Pat. No. 4,507,974 of M. L. Yelderman, issued Apr. 2, 1985; U.S. Pat. No. 4,785,823, of Eggers et al., issued on Nov. 22, 1988; and U.S. Pat. No. 5,000,190, of John H. Petre, issued on Mar. 19, 1991.
A principal limitation in the quanification of cardiac output is the existence of thermal fluctuations inherent in the bloodstream. Previous methods work with those fluctuations while observing the effects of an input signal to calculate cardiac output. The invention described herein uses a differential measurement technique to substantially eliminate the effect of the thermal fluctuations, permitting the use of a minimal thermal input signal, which allows frequent or continuous measurements.
It is desirable to obtain accurate cardiac output measurements in an effectively continuous manner, i.e., several times a minute, so that a diagnosis can be achieved more rapidly and so that rapid changes in a patient's condition can be monitored on a more continuous basis than is possible using current techniques. Moreover, it is desirable to obtain instantaneous measurements of the cardiac output on a beat-to-beat basis to evaluate the relative changes which occur from beat to beat, as well as to determine the presence of regurgitation.